1. Field of the Invention
The invention generally relates to force and/or motion measurement systems. More particularly, the invention relates to force and/or motion measurement systems with inertial compensation.
2. Background and Description of Related Art
Force measurement systems are utilized in various fields to quantify the reaction forces and moments exchanged between a body and support surface. For example, in biomedical applications, force measurement systems are used for gait analysis, assessing balance and mobility, evaluating sports performance, and assessing ergonomics. In order to quantify the forces and moments resulting from the body disposed thereon, the force measurement system includes some type of force measurement device. Depending on the particular application, the force measurement device may take the form of a balance plate, force plate, jump plate, an instrumented treadmill, or some other device that is capable of quantifying the forces and moments exchanged between the body and the support surface.
Regardless of the type of force measurement device that is employed, the device is normally positioned on a support surface. In order for the device to be accurately considered as part of an inertial system, some type of rigid connection between the force measurement device and the ground on which it is placed must exist. However, in many applications, it is either impossible and/or undesirable to rigidly affix the force measurement device to the ground on which it is supported. For example, a force measurement plate used to conduct the dynamic testing of human subjects cannot be rigidly affixed to any support surface. Consequently, the force measurement assembly will move in space, and it will measure loads due to the inertia of the force measurement components in addition to the desired externally applied loads. For force measurement assemblies that have high masses, such as instrumented treadmills, these inertia forces will be comparable to, or even higher than the externally applied loads in magnitude. In such instances, it cannot be accurately assumed that the force measurement device is part of an inertial system, and it is necessary to compensate for the forces produced by the movement of the force measurement device, which results in undesirable measurement errors.
Motion acquisition/capture systems are used in numerous fields in order to record the motion of a moving body so that the movement and forces of the body can be analyzed. In a biomedical application, such as one involving gait analysis, a plurality of markers typically are provided on the body of a subject, and the movement of these markers is recorded in 3-dimensional space using a plurality of cameras positioned at various locations within a room. Then, once the positional data is obtained using the motion acquisition/capture system, inverse kinematics are employed in order to determine the joint angles of the subject. When the computation of the joint reaction forces and joint moments of the subject is also desired, the subject is often disposed on a force measurement device so that the ground reaction forces and moments associated with the subject can be measured. These ground reaction forces and moments are used in conjunction with the joint angles computed from the inverse kinematic analysis in order to determine the net joint reaction forces and net joint moments of the subject. In particular, inverse dynamics is used to calculate the net joint reaction forces and net joint moments of the subject by using the computed joint angles, angular velocities, and angular accelerations of a musculoskeletal model, together with the ground reaction forces and moments measured by the force measurement device.
However, the net joint reaction forces and net joint moments will not be accurately determined during the inverse dynamics analysis if the force measurement device, which is employed for measuring the ground reaction forces, is in motion. In such a case, the force measurement device will measure the loads due to the inertia of the force measurement components in addition to the desired ground reaction forces, which will introduce errors in the calculations. As described above, these errors will be quite substantial for force measurement assemblies having large masses, such as instrumented treadmills. Therefore, because the inertia of the force measurement assembly will result in substantial errors in the computed net joint reaction forces and net joint moments of the subject, compensation for the inertia of the force measurement system is necessary.
What is needed, therefore, is a force and/or motion measurement system having inertial compensation that accurately corrects for the movement of the force measurement device in multiple dimensions. Moreover, a force measurement and/or motion system is needed that is capable of empirically determining the inertial parameters of a large, complex force measurement assembly. While an analytical approach can be used for simple systems wherein the motion is limited to one direction, an analytical approach will not produce sufficiently accurate results for large systems that undergo complex multi-dimensional motion. Furthermore, a need exists for a force measurement system that produces accurate measurements when the entire system is in motion.